A typical gas turbine includes an inlet section, a compressor section, a combustion section, a turbine section, and an exhaust section. The inlet section cleans and conditions a working fluid (e.g., air) and supplies the working fluid to the compressor section. The compressor section progressively increases the pressure of the working fluid and supplies a compressed working fluid to the combustion section. The compressed working fluid and a fuel are mixed within the combustion section and burned in a combustion chamber to generate combustion gases having a high temperature and pressure. The combustion gases are routed along through a hot gas path into the turbine section where they expand to produce work. For example, expansion of the combustion gases in the turbine section may rotate a shaft connected to a generator to produce electricity.
The combustion section generally includes one or more combustors annularly arranged and disposed between the compressor section and the turbine section. Various parameters influence the design and operation of the combustors. For example, gas turbine manufacturers are regularly tasked to increase gas turbine efficiency without producing undesirable air polluting emissions. The primary air polluting emissions typically produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen (NOx), carbon monoxide (CO), and unburned hydrocarbons (UHCs). Oxidation of molecular nitrogen and thus the formation of NOx in air breathing engines such as gas turbines is an exponential function of temperature. The higher the temperature of the combustion gases, the higher the rate of formation of the undesirable NOx emissions.
One way to lower the temperature of the combustion gases, thus controlling the formation of NOx, is to deploy a lean pre-mix combustion system. The lean pre-mix combustion system operates by pre-mixing the fuel and working fluid to provide a lean (or air rich) pre-mixed combustible mixture to the combustion chamber. As a result, during combustion the heat capacity or thermal capacitance of the excess air present in the air rich or lean combustible mixture allows for heat absorption within the combustion chamber, thus reducing the temperature of the combustion gases, thereby reducing the formation of NOx emissions.
One factor that determines the operability range of a lean pre-mix combustion system in the lean pre-mix mode is the temperature of the compressed working fluid as it enters the combustion chamber. For example, if the temperature of the compressed working fluid entering the combustion chamber falls below certain temperatures, a large temperature rise and a large heat release may occur within the combustion chamber and across the combustion system as the lean pre-mix combustible mixture is burned. As a result, the amplitude of various pressure pulsations within the combustor also known as combustion dynamics may be increased. In addition, the large temperature rise and heat release may also result in CO production that may exceed emissions compliance standards.
The temperature of the compressed working fluid is generally influenced by the operational mode of the gas turbine and by the local ambient temperature at an inlet to the compressor. Most lean pre-mix combustion systems are designed to operate within the lean pre-mix mode at ISO standard day conditions which in the power industry correspond to an ambient temperature of 59 degrees Fahrenheit. Generally, the local ambient temperature has the greatest effect on combustor dynamics and CO levels when the gas turbine is operated at less than full-speed/full-load conditions such as during part-load operation where the compressor is operated at less than full capacity. As a result, less thermal energy is transferred to the working fluid as it flows through the compressor to the combustor. Accordingly, a system for heating the compressed working fluid within the combustor upstream from the combustion chamber would be useful.